Photovoltaic solar cells are semiconductor junction devices which convert light energy to electrical energy. Typically, such solar cells have layered structures comprising at least four layers:
(1) an absorber-generator; PA1 (2) a collector-converter; PA1 (3) a transparent electrical contact; and PA1 (4) an opaque electrical contact.
When light is incident upon a structure comprising these elements in an appropriate relationship, it creates a voltage and an electrical current which increases as the intensity of the incident light increases.
The absorber-generator (referred to as the absorber) is a layer of semiconductor material which absorbs light (photons), creating a pair of charged particles which are negatively charged carriers (electrons) and positively charged carriers (holes). In p-type semiconductors, the electrons are minority carriers and in n-type semiconductors, the holes are minority carriers. Minority carriers are readily annihilated in the absorber by recombination with majority carriers. Accordingly, they must be transported to a region where they can be converted and survive as carriers.
The collector-converter (the collector) is a layer in electrical contact with the absorber. This layer collects minority carriers from the absorber and converts them to majority carriers, the genesis of the electric current for utilization. The lifetime of minority carriers should be sufficiently long for diffusion across the absorber to the junction of the absorber and the collector. Further, the bandgap of the absorber should be of an energy to maximize energy conversion efficiency.
The transparent contact is an electrically conductive optically transparent layer in ohmic (electrical) contact with the collector. This element is generally a metallic grid covering less than 5% of the solar cell area, which makes it effectively 95% transparent.
Finally, the opaque electrical contact is a layer on the side of the absorber-collector junction opposite the incident light, i.e., in contact with the collector completing the electrical circuit through the cell.
A criterion which must be met for all of the layers is that there should be a reasonable lattice match at all the active layer (absorber and collector) interfaces as well as a reasonable match in the coefficient of thermal expansion. This is necessary to minimize any interfacial strains that might arise during epitaxial growth. A mismatch in lattice constant can lead to the creation of interface states which may, in turn, place limits on the short circuit current and open circuit voltage through increases in carrier recombination at the absorber-collector junction if these interface states are created in the junction region. Strains can be propagated from the window-collector interface, through the collector to the collector-absorber junction where they can increase minority carrier recombination.
In the art, gallium arsenide (GaAs) has been found to be one of the most efficient semiconductor materials for solar cells. It can be doped as either an n-type or p-type semiconductor, so that the absorber and collector can be of the same material. A highly efficient solar cell has been made using gallium aluminum arsenide (either Ga.sub.1-x Al.sub.x)As or simply (GaAl)As) as a window material, as disclosed in U.S. Pat. No. 3,675,026 to Woodall, July 4, 1972. This window material (on GaAs cells) has a very good transparent over the parts of the solar spectrum which can be absorbed by gallium arsenide, has a good lattice match and a similar coefficient of thermal expansion. In preparation of solar cells and in use in terrestrial conditions, however, it is deficient as a result of its hygroscopic nature and oxygen sensitivity. It is also difficult to make an ohmic transparent contact to (GaAl)As. This requires fabrication of the solar cell in a moisture-free atmosphere and protection of the completed device. In use under atmospheric conditions, this deficiency leads to deterioration of the cell. A window material resistant to moisture, but otherwise satisfying the other requirements, would greatly increase the useful life of GaAs solar cells.